1. Field of the Invention
This invention relates to a method of removing sulfur dioxide from gas streams, such as those generated by the combustion of a sulfur-containing fuel or those produced by ore processing, such as in smelting furnaces, etc. More particularly, the present invention relates to a method of removing sulfur dioxide wherein the active agent in the removal is nahcolite.
2. Description of the Prior Art
Sulfur dioxide is one of the major pollutants from the burning of sulfur-bearing fuels, processing ores, etc. The major tonnage of sulfur dioxide is discharged to the air from steam power plants and furnaces which typically burn coal and residual oils ranging in sulfur content from one to four percent and produce a flue gas concentration of sulfur dioxide ranging from 400 to about 4,000 parts per million. Processes such as ore processing produce sulfur dioxide concentrations up to 20,000 PPM in the flue gases. It has been estimated that some 25 million tons of sulfur dioxide per year are poured into the atmosphere of the United States. This has caused a tremendous pollution problem and an onslaught of proposed techniques for reducing the sulfur dioxide content of flue gases or stack gases from burning combustible products or ore processing.
In attempts to remove the sulfur dioxide from the flue gas, a number of different processes have been proposed and tried. There are numerous suggestions that the sulfur dioxide can be removed from flue gases by washing the flue gases with aqueous solutions of alkaline materials. Unfortunately, field trials using wet scrubbers have been somewhat disappointing and the degree of sulfur dioxide removal has been discouraging. Due to the failure of such wet scrubbing systems, considerable attention has been recently directed to solid systems or other types of systems where the sulfur dioxide can be removed from the flue gas without using an aqueous scrubbing medium.
For example, it has been suggested that sulfur dioxide can be separated from a flue gas on solid absorbants, such as calcium and sodium carbonate. Because sodium carbonate reacts slowly with sulfur dioxide at normal flue gas temperatures, even more recent proposals have suggested that the flue gas can be contacted with a molten bath of sodium carbonate, with the higher temperature increasing the rate of reaction.
Still further, very recent attention has been given to the injection of fine powders into a furnace so as to react with or absorb the sulfur dioxide, removing it from the flue gas. However, reports of a maximum of only about 40 percent sulfur dioxide removal through this procedure have been made. One of the principal materials proposed for such injection is limestone powder, with some limestones being more effective than others. A further material which has been suggested for the removal of sulfur dioxide from a gas stream is naturally occurring trona. Note, for example, U.S. Pat. No. 3,823,676 to Cook.
The removal of sulfur dioxide and particulate matter from flue gases through the use of an alkali metal bicarbonate has been suggested in U.S. Pat. Nos. 3,505,008 and 3,589,863 to Ludo K. Frevel and Leonard J. Kressley. The former suggests the removal of fly ash and sulfur dioxide from a gas stream by flowing the gas stream through a thin porous bed containing alkali metal bicarbonate crystalline solids having a mesh size predominantly within the range of from about 20 to about 120 mesh. The latter patent suggests the removal of sulfur dioxide and particulate matter from a gaseous stream by contacting the gaseous stream with porous alkali metal bicarbonate aggregates. While each of these methods is somewhat effective in the removal of sulfur dioxide from a flue gas, neither method is sufficiently effective to be commercially attractive.
Co-pending application Ser. No. 559,930 filed Mar. 19, 1975 in the name of John H. Knight and entitled Thermal Crushing of Alkali Compounds, discloses a method by which the sulfur dioxide content of a flue gas is reduced by introducing into the flue gas containing sulfur dioxide or other gas impurities a thermally crushed alkali compound, preferably thermally crushed nahcolite. In accordance with the disclosure of this application, the thermally crushed powder is formed in a separate thermal crushing zone, with the thermally crushed powder being transported to a further contact zone for contact with cooled flue gas at a temperature up to 1,500.degree. F. While the process set forth in this co-pending application is effective in the removal of 90 percent or more of the sulfur dioxide content of the flue gas, this method suffers from an economic disadvantage, due to the requirement of a thermal crushing zone separate and apart from the boiler, furnace, incinerator, or other unit providing a high temperature zone and the necessary transporting of the thermally crushed powder to a further zone for contact with the flue gas.